Many of the optical disk apparatuses, including DVD drives, currently in use employ a high-frequency modulation method to reduce noise generated by a laser diode used as a laser light source. This is commonly known among those in the art. In terms of the high-frequency modulation method, therefore, only aspects associated with the present specification will be described in the following only to a required extent without further elaboration.
The background of the invention will be described below with reference to the inventions disclosed in JP-A No. H8 (1996)-77640, JP-A No. H8 (1996)-221758 and JP-A No. 2002-230814, or the reference—Lathi, B. P., Modern Digital and Analog Communication Systems, Volume 1, HBJ, 1985, p. 223.
In the high-frequency modulation method, the laser diode used is pulsed. Namely, the laser light intensity waveform obtained from the laser diode represents alternating emission-on and emission-off states as shown in FIG. 2. The duty that is the ratio of laser pulse duration to laser pulse interval (associated with laser pulse frequency) is a parameter which is adjusted to minimize laser noise. When a laser light intensity waveform is as shown in FIG. 2, a corresponding read signal waveform becomes as shown in FIG. 3 provided that no bandwidth limitation is effected either by a read photodiode or by a current to voltage converter amplifier. A signal composed of such a train of read pulses will hereinafter be referred to as a pulse read signal.
In FIG. 3, the dashed line represents a read signal waveform obtained when a laser diode is made to emit continuously with an output equivalent to a laser pulse peak obtained by high-frequency modulation. Namely, the upper envelope of the pulse read signal represents a read signal waveform obtained from continuous light. It is, therefore, possible to obtain a desired read signal waveform by making the pulse read signal go through a low-pass filter with a cutoff frequency adequately lower than the superimposed HF frequency (high frequency). In optical disk apparatuses currently in use, this is realized through bandwidth limitation by a system composed of a photodetector and a current to voltage converter amplifier and an analog equalizer. As for the HF frequency to be superimposed, in the case of a Blu-ray Disc (hereinafter referred to as the “BD”) system, for example, it is typically about 400 MHz. The HF frequency to be superimposed is determined solely by the type of laser to be used and the optical path length of the read optical system, so that it does not differ much between apparatuses. The upper limit of the read signal frequency band based on a basic read speed (1×) is 16.5 MHz with the shortest recording mark or space being 2T (T=channel clock period).
FIG. 4A shows an example of a read signal spectrum obtained when a 25 GB disc is read at 6× speed in a BD system. FIG. 4B shows an example of a spectrum of HF laser pulses. The spectrum is an emission-line spectrum with the HF laser pulses making up a periodic signal. The HF frequency is set at 396 MHz to be equal to the channel clock frequency, i.e. six times the upper limit of the read signal spectrum. In this example, the laser pulse duty is 25%. The emission-line spectrum intensity ratio between spectral orders changes depending on the duty. FIG. 4C shows a pulse read signal spectrum. The waveform of the pulse read signal represents the product of the read signal waveform represented by the dashed line in FIG. 3 and the periodic HF pulse train, so that the pulse read signal spectrum is a convolution of the read signal spectrum and the pulse light spectrum. The pulse read signal spectrum is characterized in that the intensity of a higher-order read signal is comparable to that of the baseband read signal spectrum and that it has an emission line-like spectrum. Therefore, extracting a read signal, i.e. a baseband read signal spectrum, using bandwidth limitations by a system composed of a photodetector and a current to voltage converter amplifier and an analog equalizer requires the upper limit frequency of the read signal and the HF frequency to be adequately separated from each other.
Laser diodes used as laser light sources in optical disk apparatuses used to pose problems of not being adequately stable and generating large laser noise. The high-frequency modulation method is used to address such problems. It can reduce laser noise by pulsing the laser under appropriate conditions.
Even though, in the high-frequency modulation method, laser noise can be reduced to some extent, a resultant read signal obtained still contains a non-negligible amount of laser noise components. Attempts have been made, as disclosed in detail in JP-A No. 2002-183970 or in the reference—Kobayashi, M, et al., “Blu-ray Disc KOUGAKUKEI NI OKERU SHINGOU SHORI GIJYUTSU” (Technique for Signal Processing in Blu-ray Disc Optical System), Technical Report of IEICE CPM2003-100 (September 2003), item 29-34. In the method used in such attempts, a laser noise waveform is monitored using a power monitor system having an adequately wide bandwidth, and the laser noise waveform monitored is subtracted from the read signal.
FIG. 5 schematically shows the configuration of a laser noise elimination circuit for the above method. In the power monitor system shown in FIG. 5, a portion of a laser output beam is extracted at a beam splitter 42 from the main path, and the extracted beam is converged on a photodiode 9′ using a conversion lens 8′ and the intensity of the converged beam is measured. Conventionally, a power monitor was, in many cases, used to measure an average laser power, so that its bandwidth was normally narrower than the read signal bandwidth. In the present known example, the photodiode 9′ and the current amplifier 13′ connected to the photodiode 9′ have adequately wide bandwidths making it possible to monitor not only the laser power but also a laser noise waveform.
In a read signal, laser noise is not simply superimposed on the read signal. The laser noise amplitude in the read signal differs between marks and spaces. This is because the laser noise, in reality, represents light source intensity modulation. In the present known example, the measured laser noise is not simply subtracted from the read signal, but, as known from FIG. 5, analog operation is performed to eliminate the influence of the laser noise. First, the read signal and the power monitor output having passed a low-pass filter 15 are multiplied using an analog multiplier 40. Next, the power monitor output having passed a high-pass filter 41 and the read signal are multiplied at the analog multiplier 40′. The product obtained is subtracted from the product of the first multiplication at a subtractor 30. Through this process, the influences of the amplitude difference between the read signal and the power monitor output as well as that of the recording marks are eliminated, that is, the laser noise is eliminated from the read signal.